Easy choice, keep it the way it is and don't go over 50. Leave it stock and enjoy it. Next choice, a later 235 or 261 with full pressure oiling and insert bearings. Can be hopped up. For happier hiway cruising install 3.55 gears from a 1950 - 54 Powerglide model or swap in the whole rear axle. Or, you can buy the gears new. A later six, or a V8 WILL fit your present drive train if you go about it right. Both use the same (manual trans) bellhousing. This bell will fit a torque tube drive trans IF it is the right one. Usually found in 1942 - 54 1/2 ton pickups. There are 2 different trans cases but they both use all the same internal parts. Requires slight grinding of the bellhousing for clearance. The difference is the bottom bolts on the 'good' case are wider apart. It might be possible to drill the bellhousing for the other trans case but I don't know anyone who has done this. As others pointed out, the newer six is longer and requires different mounts, moving the rad forward and a few other mods. V8 not recommended with the stock drive train unless it is small and low powered like a stock 283 or 305 and you baby it. The V8 is a tight squeeze, the six a lot easier to fit. So you have some choices and you don't have to gut your car like a trout and install a whole new drive train if you don't want to. As for my personal choice - I just bought a 261 out of a 62 Canadian Pontiac for my 42 Chevy.